The introduction in recent years of optical amplifiers with wide wavelength operating range in the 1520 to 1580 nm spectral region into fiber communication systems has enabled the practice of dense wavelength division multiplexing (DWDM). In DWDM systems a group of sixteen or even as many as sixty four wavelengths are simultaneously transmitted in a fiber, each wavelength being modulated by a data stream as fast as 10 Gb/s. Such high capacity communication systems consist of optical transmitters, cabled fiber, optical amplifiers, wavelength multiplexers and optical receivers and need to be closely monitored to detect any malfunction that may corrupt the information transmission. The bit error rate (BER) is a measure defined as the number of errored bits divided by the total number of bits received in a given time period. BER is sensitive to several parameters of the transmission system such as the optical power at the receiver, the quality of the transmitter, but particularly to the ratio of signal power to noise power, known as the signal to noise ratio (SNR) at the receiver. In turn the SNR is determined by the addition of receiver thermal noise shot noise and noise added by optical amplifiers in the system.
One of the most important parameters of optical amplifiers is the gain at the various wavelengths. For proper operation the receivers operating at the various wavelengths expect a common and substantially equal optical signal to noise ratio as well as substantially equal optical power. Since transmitters generally output substantially equal amounts of power at various wavelengths, the amplifiers in the system are expected to provide equal gain at the various channel wavelengths.
Several methods of equalizing or flattening the gain of optical amplifiers have been explored in the literature. The book titled "Erbium Doped Optical Amplifiers" by Emmanuel Desurvire discusses such gain flattening techniques on page 480. The article "Flat-gain amplifiers and transmission in WDM networks", paper FA1-1 presented at the "Optical amplifiers and their applications" conference by Bertrand Clesca also compares gain flattening techniques. In Erbium doped fibers the gain at any given wavelength has contributions from several broadened atomic resonances. Each of these resonances is centered at a slightly different wavelength in the 1520 to 1580 nm range and originates from a different pair of Stark sublevels in Erbium. The material of the glass matrix containing the Erbium as well as dopants in that matrix, affect the position and broadening of the atomic resonances in Erbium. One of the known techniques for flattening the gain curve is the use of aluminum co-doping of Erbium doped fiber. Another technique utilizes fluoride glass instead of silica glass as the fiber material. Yet other techniques insert specially shaped spectral filters in line with the amplifiers to compensate for the difference in gain at different wavelengths. U.S. Pat. No. 5,050,949 by Di Giovanni and Giles, describes the use of two stage fiber amplifiers to achieve flattened gain. The drawback of the two stage approach is that it still lacks enough suppression of the gain in the 1520 to 1535 nm spectral region to achieve the desired degree of flatness.
More recently, U.S. Pat. No. 5,557,442 by Huber describes a technique involving the use of a circulator, a series of fiber Bragg reflectors imprinted into a fiber and a set of attenuators to achieve gain flattening. Circulators are optical devices with three or more fiber ports that channel the light from port I into port I+1, Bragg gratings are periodic index of refraction gratings imprinted into the fiber core by UV light and reflect fiber propagating light at specific wavelengths matching the periodicity of the gratings. In U.S. Pat. No. 5,557,442 the light enters the amplifier then the circulator and is channeled into the chain of Bragg reflectors and attenuators. The light at a first wavelength W1 which experiences little gain in the amplifier is reflected from the first Bragg grating and thus suffers no attenuation before returning to the circulator and being channeled out. Light at a second wavelength W2 which experiences somewhat higher gain than W1 is reflected by a subsequent Bragg grating thus having to go through one or more attenuators before it returns to the circulator and gets channeled out. Light at a wavelength W3 experiencing the highest gain in the amplifier is reflected back by the last Bragg grating in the fiber thus forcing it to go through the whole attenuator chain twice before it reaches the circulator and gets channeled out.
Several drawbacks of U.S. Pat. No. 5,557,442 are apparent. The multitude of attenuators required adds to the cost of the device and to the complexity of its construction since such attenuators are generally bulk optical devices which include lenses and involve stringent alignment requirements in manufacturing. Furthermore such attenuators may exhibit temperature dependence and are required to be spliced between the Bragg gratings one at a time. Finally the Huber device allows unidirectional operation only.
Recently it has become generally accepted that optical amplifiers allowing bidirectional transmission in WDM networks offer several advantages. The advantages of bidirectional amplifiers and a specific embodiment of a bidirectional amplifier are described in U.S. Pat. No. 5,633,741 by Giles. Giles teaches the use of two four-port circulators in conjunction with fiber gratings to achieve a bidirectional amplifier.
A generally useful device in fiber optics whose function is to transfer or channel light from one fiber to the next fiber in an ordered set of fibers is known as the circulator. For example a circulator device will channel light from the first fiber in a set to the second fiber and from the second fiber to the third. The point of connection between a fiber and the circulator is referred to as a port and generally the ports are numbered to indicate their ordering. Circulators with three and four ports are commercially available from companies such as Etek of San Jose, Calif. and The Kaifa Group of Sunnyvale, Calif.
Referring to FIG. 1, light at frequencies f2 and f4 enters the first port P1 of the first circulator C1 and is channeled to the second port P2 where it enters a fiber with Bragg gratings G2 configured to reflect at f2 and f4 back into the circulator C1 and out to the third port P3 which is connected to the amplifier A1. The output of the amplifier A1 is then fed to the fourth port P4 of a second circulator C2 and from there f2 and f4 exit through the first port P1 of the second circulator C2. The wavelengths or frequencies f1 and f3 propagating in opposition to f2 and f4 are input through the first port P1 of the second circulator C2 and follow a similar if reverse path to f2 and f4. Other types of bidirectional amplifiers have been described as in U.S. Pat. No. 5,604,627 to Kohn where two wavelength division multiplexing devices (WDM) are utilized in place of the circulators to separate the counter-propagating light streams.
The drawback of the devices shown in the prior art is their failure to provide flat gain profiles especially in the 1533 nm spectral range as well as bidirectional operation. It is therefore a first objective of this invention to provide an erbium doped amplifier with a flat gain profile which is capable of bidirectional operation.
Another objective of this invention is to simplify the construction of the amplifier and to reduce the cost of its manufacturing.
A third objective is to eliminate the use of attenuators and their temperature dependence in either unidirectional or bidirectional amplifiers.
A fourth objective is to reduce the number of circulators and circulator ports used.
A fifth objective is to eliminate the need for fiber couplers or wavelength routers required in the prior art to couple the pump and signal lights into the doped fiber.
It is a further objective of this invention to remedy the general drawbacks described in the background section.